LOANS - Allowance for Loan Losses (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	$ 38,189	$ 208,793	$ 31,793	$ 38,189	$ 28,819	$ 28,819
Provision for loan losses		26,692	5,989	132,188	14,065
Loans charged off		(7,371)	(5,319)	(25,370)	(14,194)
Recoveries of loans previously charged off		3,810	3,067	8,256	6,840
Ending balance, Allowance		231,924	35,530	231,924	35,530	38,189	$ 28,819
Period-end allocation:
Loans individually evaluated for impairment									$ 7,162	$ 6,451
Loans collectively evaluated for impairment									31,027	29,079
Ending balance, Allowance	$ 38,189	208,793	35,530	$ 38,189	35,530	$ 28,819	$ 28,819	$ 231,924	38,189	35,530
Loans:
Individually evaluated for impairment									82,475	60,871
Collectively evaluated for impairment									12,609,278	12,582,406
Acquired with deteriorated credit quality									126,723	183,007
Total								14,943,593	12,818,476	12,826,284
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member			us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
Nonaccrual loan								138,163	75,124
Minimum | Substandard
Loans:
Nonaccrual loan								100	100	100
Troubled debt restructuring loans								100	100	100
Accounting Standards Update 2016-13
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	$ 78,661			$ 78,661
Ending balance, Allowance	78,700					$ 78,661
Period-end allocation:
Ending balance, Allowance	78,661			78,661		78,661			78,661
Commercial, financial and agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	4,567	7,938	2,816	4,567	2,352	2,352
Provision for loan losses		5,533	831	8,624	1,848
Loans charged off		(1,715)	(490)	(4,687)	(1,647)
Recoveries of loans previously charged off		470	300	1,135	904
Ending balance, Allowance		12,226	3,457	12,226	3,457	4,567	$ 2,352
Period-end allocation:
Loans individually evaluated for impairment									1,543	1,022
Loans collectively evaluated for impairment									3,024	2,435
Ending balance, Allowance	4,567	12,226	3,457	12,226	2,352	2,352	2,352	12,226	4,567	3,457
Loans:
Individually evaluated for impairment									8,032	4,176
Collectively evaluated for impairment									789,252	918,162
Acquired with deteriorated credit quality									4,887	9,417
Total								1,879,788	802,171	931,755
Nonaccrual loan								11,844	9,236
Commercial, financial and agricultural | Accounting Standards Update 2016-13
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	2,587			2,587
Ending balance, Allowance						2,587
Period-end allocation:
Ending balance, Allowance	2,587			2,587		2,587			2,587
Consumer installment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	3,784	20,085	3,388	3,784	3,795	3,795
Provision for loan losses		(3,693)	1,132	5,943	3,289
Loans charged off		(677)	(1,245)	(2,781)	(4,313)
Recoveries of loans previously charged off		516	476	1,273	980
Ending balance, Allowance		16,231	3,751	16,231	3,751	3,784	3,795
Period-end allocation:
Loans individually evaluated for impairment									0	0
Loans collectively evaluated for impairment									3,784	3,751
Ending balance, Allowance	3,784	16,231	3,751	3,784	3,751	3,795	3,795	16,231	3,784	3,751
Loans:
Individually evaluated for impairment									0	0
Collectively evaluated for impairment									498,363	500,067
Acquired with deteriorated credit quality									214	0
Total								450,810	498,577	500,067
Nonaccrual loan								947	831
Consumer installment | Accounting Standards Update 2016-13
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	8,012			8,012
Ending balance, Allowance						8,012
Period-end allocation:
Ending balance, Allowance	8,012			8,012		8,012			8,012
Indirect automobile
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	0	3,381	0	0	0	0
Provision for loan losses		322	580	1,138	580
Loans charged off		(697)	(965)	(2,944)	(965)
Recoveries of loans previously charged off		317	385	1,020	385
Ending balance, Allowance		3,323	0	3,323	0	0	0
Period-end allocation:
Loans individually evaluated for impairment									0	0
Loans collectively evaluated for impairment									0	0
Ending balance, Allowance	0	3,323	0	0	0	0	0	3,323	0	0
Loans:
Individually evaluated for impairment									0	0
Collectively evaluated for impairment									1,056,811	1,108,717
Acquired with deteriorated credit quality									5,013	3,098
Total								682,396	1,061,824	1,111,815
Nonaccrual loan								1,936	1,746
Indirect automobile | Accounting Standards Update 2016-13
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	4,109			4,109
Ending balance, Allowance						4,109
Period-end allocation:
Ending balance, Allowance	4,109			4,109		4,109			4,109
Mortgage warehouse
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	640	1,498	640	640	640	640
Provision for loan losses		498	0	893	0
Loans charged off		0	0	0	0
Recoveries of loans previously charged off		0	0	0	0
Ending balance, Allowance		1,996	640	1,996	640	640	640
Period-end allocation:
Loans individually evaluated for impairment									0	0
Loans collectively evaluated for impairment									640	640
Ending balance, Allowance	640	1,996	640	1,996	640	640	640	1,996	640	640
Loans:
Individually evaluated for impairment									0	0
Collectively evaluated for impairment									526,369	562,598
Acquired with deteriorated credit quality									0	0
Total								995,942	526,369	562,598
Mortgage warehouse | Accounting Standards Update 2016-13
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	463			463
Ending balance, Allowance						463
Period-end allocation:
Ending balance, Allowance	463			463		463			463
Municipal
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	484	507	502	484	509	509
Provision for loan losses		365	(5)	480	(12)
Loans charged off		0	0	0	0
Recoveries of loans previously charged off		0	0	0	0
Ending balance, Allowance		872	497	872	497	484	509
Period-end allocation:
Loans individually evaluated for impairment									0	0
Loans collectively evaluated for impairment									484	497
Ending balance, Allowance	484	872	497	484	509	509	509	872	484	497
Loans:
Individually evaluated for impairment									0	0
Collectively evaluated for impairment									564,304	578,267
Acquired with deteriorated credit quality									0	0
Total								725,669	564,304	578,267
Municipal | Accounting Standards Update 2016-13
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	(92)			(92)
Ending balance, Allowance						(92)
Period-end allocation:
Ending balance, Allowance	(92)			(92)		(92)			(92)
Premium finance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	2,550	8,198	2,984	2,550	1,426	1,426
Provision for loan losses		(2,316)	1,141	235	3,224
Loans charged off		(1,159)	(1,267)	(3,893)	(3,452)
Recoveries of loans previously charged off		1,224	736	2,584	2,396
Ending balance, Allowance		5,947	3,594	5,947	3,594	2,550	1,426
Period-end allocation:
Loans individually evaluated for impairment									758	1,606
Loans collectively evaluated for impairment									1,792	1,988
Ending balance, Allowance	2,550	5,947	3,594	5,947	3,594	1,426	1,426	5,947	2,550	3,594
Loans:
Individually evaluated for impairment									6,768	2,428
Collectively evaluated for impairment									647,901	654,142
Acquired with deteriorated credit quality									0	0
Total								710,890	654,669	656,570
Nonaccrual loan								0	600
Premium finance | Accounting Standards Update 2016-13
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	4,471			4,471
Ending balance, Allowance						4,471
Period-end allocation:
Ending balance, Allowance	4,471			4,471		4,471			4,471
Real estate – construction and development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	5,995	54,266	4,763	5,995	4,210	4,210
Provision for loan losses		(10,208)	(182)	25,379	254
Loans charged off		(9)	0	(83)	(268)
Recoveries of loans previously charged off		182	930	692	1,315
Ending balance, Allowance		44,231	5,511	44,231	5,511	5,995	4,210
Period-end allocation:
Loans individually evaluated for impairment									204	552
Loans collectively evaluated for impairment									5,791	4,959
Ending balance, Allowance	5,995	44,231	5,511	44,231	4,210	4,210	4,210	44,231	5,995	5,511
Loans:
Individually evaluated for impairment									1,605	7,810
Collectively evaluated for impairment									1,532,786	1,441,918
Acquired with deteriorated credit quality									14,671	18,968
Total								1,628,255	1,549,062	1,468,696
Nonaccrual loan								5,666	1,988
Real estate – construction and development | Accounting Standards Update 2016-13
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	12,248			12,248
Ending balance, Allowance						12,248
Period-end allocation:
Ending balance, Allowance	12,248			12,248		12,248			12,248
Real estate – commercial and farmland
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	9,666	83,593	8,676	9,666	9,659	9,659
Provision for loan losses		18,419	1,541	72,410	2,283
Loans charged off		(2,977)	(1,315)	(10,220)	(3,158)
Recoveries of loans previously charged off		904	74	1,010	192
Ending balance, Allowance		99,939	8,976	99,939	8,976	9,666	9,659
Period-end allocation:
Loans individually evaluated for impairment									953	1,044
Loans collectively evaluated for impairment									8,713	7,932
Ending balance, Allowance	9,666	99,939	8,976	9,666	8,976	9,659	9,659	99,939	9,666	8,976
Loans:
Individually evaluated for impairment									19,759	16,120
Collectively evaluated for impairment									4,256,397	4,090,640
Acquired with deteriorated credit quality									76,883	91,999
Total								5,116,252	4,353,039	4,198,759
Nonaccrual loan								39,248	23,797
Real estate – commercial and farmland | Accounting Standards Update 2016-13
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	27,073			27,073
Ending balance, Allowance						27,073
Period-end allocation:
Ending balance, Allowance	27,073			27,073		27,073			27,073
Real estate – residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	10,503	29,327	8,024	10,503	6,228	6,228
Provision for loan losses		17,772	951	17,086	2,599
Loans charged off		(137)	(37)	(762)	(391)
Recoveries of loans previously charged off		197	166	542	668
Ending balance, Allowance		47,159	9,104	47,159	9,104	10,503	6,228
Period-end allocation:
Loans individually evaluated for impairment									3,704	2,227
Loans collectively evaluated for impairment									6,799	6,877
Ending balance, Allowance	10,503	$ 47,159	$ 9,104	47,159	$ 6,228	6,228	$ 6,228	47,159	10,503	9,104
Loans:
Individually evaluated for impairment									46,311	30,337
Collectively evaluated for impairment									2,737,095	2,727,895
Acquired with deteriorated credit quality									25,055	59,525
Total								2,753,591	2,808,461	$ 2,817,757
Nonaccrual loan								$ 78,522	36,926
Real estate – residential | Accounting Standards Update 2016-13
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance, Allowance	19,790			19,790
Ending balance, Allowance						19,790
Period-end allocation:
Ending balance, Allowance	$ 19,790			$ 19,790		$ 19,790			$ 19,790